There are known conventional image acquisition apparatuses provided with an illuminating means for radiating light of two wavelengths that have different absorption intensities in blood (for example, see PTLs 1 and 2).
According to such image acquisition apparatuses, an image in which the shapes of blood vessels are extracted by radiating narrow-band light having hemoglobin absorption wavelengths and an image that is acquired when narrow-band light in a wavelength band other than the hemoglobin absorption wavelengths is radiated are separately acquired, thus making it possible to observe a buildup of capillaries etc. in tissue, such as mucosa, without staining.